`SftpSession.write()` is not thread-safe

[artembilan]

Discussed in #9793

^{Originally posted by vnxvd January 27, 2025}
Hello everybody,
I am getting following error message while using Spring Integration Sftp.

WritePendingException: A write operation is already pending; cannot write 6905 bytes, stackTrace: 
org.apache.sshd.common.io.WritePendingException: A write operation is already pending; cannot write 6905 bytes 
at org.apache.sshd.common.channel.ChannelAsyncOutputStream.writeBuffer(ChannelAsyncOutputStream.java:110) 
at org.apache.sshd.sftp.client.impl.DefaultSftpClient.write(DefaultSftpClient.java:308) 
at org.apache.sshd.sftp.client.impl.SftpOutputStreamAsync.internalFlush(SftpOutputStreamAsync.java:360) 
at org.apache.sshd.sftp.client.impl.SftpOutputStreamAsync.flush(SftpOutputStreamAsync.java:293) 
at org.springframework.util.FileCopyUtils.copy(FileCopyUtils.java:112) 
at org.springframework.integration.sftp.session.SftpSession.write(SftpSession.java:140)

This is the code for creating Sftp Session Factory.

DefaultSftpSessionFactory sessionFactory = new DefaultSftpSessionFactory(true);
        sessionFactory.setHost(connection.getHost());
        if (connection.getPort() != null) {
            sessionFactory.setPort(connection.getPort());
        }
        sessionFactory.setUser(connection.getUser());
        if (StringUtils.isNotBlank(connection.getPassword())) {
            sessionFactory.setPassword(connection.getPassword());
        } else if (StringUtils.isNotBlank(connection.getPrivateKey())) {
            sessionFactory.setPrivateKey(new FileSystemResource(connection.getPrivateKey()));
            if (StringUtils.isNotBlank(connection.getPrivateKeyPassphrase())) {
                sessionFactory.setPrivateKeyPassphrase(connection.getPrivateKeyPassphrase());
            }
        }
        sessionFactory.setAllowUnknownKeys(true);
        return sessionFactory;

This is the code for creating Sftp Session and use the session.

SftpSession sftpSession = sessionFactory.getSession();
        if (!sftpSession.exists(targetFile)) {
            try (InputStream is = new FileInputStream(sourceFile)) {
                sftpSession.write(is, targetFile);
            } catch (IOException e) {// handle exception here}
        }

Library versions:
Spring Boot 3.4.0
Spring Integration Sftp 6.4.0
Apache sshd Sftp 2.14.0
As a temporary solution, I am using "synchronized" keyword on the method which using the SftpSession object to fix the error. Please let me know if you have a better idea to fix this error message.
Thanks.